A high frequency lamp ballast is essentially a single-phase ac-to-ac converter, which draws the low frequency power from the utility grid and converts it into a high frequency ac power to feed the lamp. As lighting equipments represent a significant portion of the total load at many installations, maintaining a high power factor is becoming more and more important for the lamp ballasts.
High input power factor is required to reduce the rms line current and its total harmonic distortions so that the utility power line can be more efficiently utilized and less polluted. As a consequence, a current-shaping or power factor correction stage has to be added at the input to conventional ballasts. That makes a typical high power factor lamp ballast consist of two cascaded processing stages. The first stage is designed to be a high power factor rectifier (effectively an ac-to-dc converter), and the second stage is then designed to be a high frequency switching dc-to-ac converter to provide the requisite high frequency ballast function. Both passive and active means can be used in the implementation of the input stage to shape the input current, but because the ballast then consists of two stages in cascade the ballast operates with reduced efficiency and suffers from increased size and weight, thus doubling the cost and reduced reliability.
The resonant matching network for a fluorescent lamp has the following twofold function. It will provide the lamp with the desirable low crest factor sinewave current and presents current source characteristics which provide high enough voltage to strike the lamp during ignition, and it will also stabilize its running current after the lamp strikes as required owing to the fluorescent lamps negative impedance characteristic.
To implement a ballast topology, a "squarewave" switching converter is preferred as disclosed in U.S. Pat. No. 5,416,387 recently granted to the present inventors, because it results in simple control and small component count, with several reactive components to form a resonant matching network that shapes the high frequency ac waveform and provides the high output impedance required by the lamp. The resonant matching network also provides the sinusoidal lamp current with a low crest factor of 1.4.
Two basic squarewave topologies for ballasts are half-bridge and pushpull resonant converters. The halfbridge topology shown in FIG. 1a comprises MOSFET power transistors Q1 and Q2 that are switched ON alternately to provide a squarewave voltage across a resonant matching network, which comprises and inductor L and capacitors Cs and Cp. That LCC network properly designed is the most popular resonant matching network when the halfbridge topology is used. The voltage and current waveforms of the switching devices are squarewave and sinusoidal, respectively, as shown in FIG. 1b. Hence the LCC ballast experiences a relatively low voltage stress and a high current stress.
Since the resonant matching network provides a stable operating point for the lamp, the ballast can be driven in a simple open-loop manner. As noted above with reference to FIG. 1a, the most popular resonant matching network is an LCC network where a series capacitor Cs of the LCC network is provided to block the dc component of the input pulsing voltage, while an inductor L together with a shunting capacitor Cp and the series capacitor Cs form a series-parallel resonant circuit which provides a current source output at or near its parallel-resonant frequency. If the input voltage to the resonant matching network does not contain any dc component, then a simple L, Cp parallel resonant network can be used.
The pushpull topology is essentially the dual form of the halfbridge converter, where a dc current is chopped alternately by two active switches Q1 and Q2 configured in the pushpull manner shown in FIG. 2a. A center tapped transformer T is necessary to provide ac power with a voltage stepup. Usually a resonant capacitor Cp is placed across the transformer primary to shape the waveform, and a capacitor Cb is placed in series with the lamp to limit the lamp current. As opposed to the ballast of FIG. 1a, current through the switch Q1 is basically dc while voltage across the switch is a rectified sinewave whose peak value is at least n times the input voltage Vdc as shown in FIG. 2b.
A lamp ballast based on a class-E tuned power amplifier using a MOSFET switch and an LCC resonator (referred to hereinafter as a class-E converter) was introduced in the prior art by G. Lutteke, II and C. Raets in papers titled "High Voltage High Frequency Class-E Converter Suitable for Miniaturization," IEEE Power Electronics Specialists Conference, 1984, pp. 54-61 and "220V Mains 500 kHz Class-E Converter Using a Bimos," IEEE Power Electronics Specialists Conference, 1985, pp. 127-135. They allow for high efficiency switching at high frequencies for use in a lamp ballast owing to zero switching losses.
In order to satisfy the need for high performance and low cost, a family of single stage high power factor converters, disclosed by A. Hiramatsu, K. Yamada, F. Ohamoto and M. Mitani in a paper titled "Low THD Electronic Ballast with a new AC-DC-converter operation," presented at an August, 1992 conference of the Illuminating Society of North America, combine a power factor correction boost converter and a half-bridge converter. A single-ended type single-switch converter was also disclosed based on the combination of a boost converter and a class-E converter as shown in FIG. 3. However, the boost converter has a voltage conversion ratio larger than 1. When it is operated in the discontinuous inductor current mode (DICM), its output voltage has to be at least twice the peak input (line) voltage so as to maintain a power factor close to unity, which imposes an unreasonably high voltage stress on the single active switching device when the class-E converter operation is also taken into account. A practical implementation of another single switch circuit is presented in a paper by L. Malesani, L. Rosseto, G. Siazzi and P. Tenti, "High Efficiency Electronic Lamp Ballast With Unity Power Factor," IEEE Industry Applications Society (IAS) Annual Conference, 1992, pp. 681-688. It proposes to reduce voltage stress at the cost of losing zero-voltage switching.
FIG. 4 illustrates a single-stage, dual-switch, high power factor lamp ballast disclosed in U.S. Pat. No. 5,416,387. Owing to a diode D1, it operates in the discontinuous inductor current mode (DICM). That diode decouples the automatic input current-shaping function from the high frequency output lamp ballasting function. The lamp ballast thus based on the Cuk converter first disclosed in U.S. Pat. No. 4,184,197 is similar to but has advantages over the prior art discussed above, such as high power factor and automatic current shaping as well as naturally provided soft switching. However, it requires two active switches Q1 and Q2, thus adding some complexity to the circuit.